1. Field of the Invention
The present invention relates to the field of thermophotovoltaic (TPV) direct energy conversion in general, and in particular this invention embodies a novel method to improve efficiency by coating the emitter with multiple layers of oxides and molybdenum.
2. Description of the Related Art
A thermophotovoltaic (TPV) system consists of a radiator surface that is heated to produce photons that are converted to electrical power by TPV cells. High temperature metallic structural materials that are considered for TPV radiator applications, such as molybdenum, have a low emissivity (xcx9c0.1 to 0.2) and do not provide the needed power density for a practical TPV system.
Two approaches have been taken for increasing the surface emissivity of a molybdenum TPV radiator to 0.8 or higher for at least 500 hours at 1100 degrees C. The first approach involves application of a thermal spray coating of either a carbide or an oxide and is discussed in B. V. Cockeram, D. P. Measures, and A. J. Mueller, The development and testing of emissivity enhancement coatings for thermophotovoltaic (TPV) radiator applications, Thin Solid Films Vol. 355/356 (1999) at 17-25, and B. V. Cockeram and J. L. Hollenbeck, The spectral emittance and stability of coatings and texture surfaces for thermophotovoltaic (TPV) radiator applications, Elevated Temperature Coatings: Science and Technology IV, N. B. Dahotre, J. M. Hampikian, and J. E. Moral (eds.), TMS, Warrendale, Pa. (2001) at 327-342. Other examples of use of oxide coatings to improve efficiency are described in Fraas, et al., U.S. Pat. No. 6,177,628 and Fraas, et al., U.S. Pat. No. 5,403,405. The second approach, also discussed in the references cited above, involves surface modification to create a rough surface texture.
Both the coating and surface modification candidates are essentially greybody emitters that provide a high emittance over a wide range of wavelengths, see FIG. 1. Photons emitted from a greybody radiator with a wavelength of less than 2 to 2.5 microns (energy greater than or equal to bandgap energy of the TPV cell (Eg=0.52 to 0.55 eV)) are converted to electrical power. Photons emitted from a greybody radiator with a wavelength greater than 2 to 2.5 microns (energy less than Eg) are not converted into electrical power, and must be filtered or reflected back to the radiator.
This invention describes the use of multilayer coatings for a molybdenum TPV radiator that selectively emits photons predominantly in the wavelength range (1 to 3 microns) that matches the bandgap of TPV cells with Eg=0.52 to 0.55 eV to provide the power density that is needed for a practical TPV system. A low emissivity is provided by these selective emitting coatings at wavelengths greater than 3 microns (energy less than Eg), which greatly improves the efficiency of a TPV system.
Multilayer coating designs have been used to increase or control the absorptance of various materials in the solar spectrum range (wavelength less than 1 micron). See, e.g., D. A. Jaworkske, Thin Solid Films, Vol. 332 (1998) at 30-33; R. E. Hahn and B. O. Seraphin, J. Vac. Sci. Technology, Vol 12. (1975) at 905; R. E. Hahn and B. O. Seraphin, xe2x80x9cSpectrally Selective Surfaces,xe2x80x9d (1979) at 1-69; R. E. Peterson and J. R. Ramsey, J. Vac. Sci. Technology, Vol. 12 (1975) at 471; and R. N. Schmidt, K. C. Park, and J. E. Janssen, xe2x80x9cHigh Temperature Solar Absorber Coatings,xe2x80x9d WPAFB Tech. Rep. No. ML-TDR-64-250, Part I (1963) and Part II (1964).
A 3-layer Al2O3/Molybdenum/Al2O3 coating was developed for molybdenum that provided high emittance at a wavelength less than 1 micron. No other selective emitting coatings have been developed for molybdenum. The 3-layer Al2O3/Molybdenum/Al2O3 coatings developed here are significantly different because selective emission is provided over the spectral range that matches the bandgap of the TPV MIM cells (wavelength=1 to 3 microns), which is functionally different from the optimum spectral range (solar spectrum; wavelength less than 1 micron) of the coatings described in R. E. Peterson and J. R. Ramsey, J. Vac. Sci. Technology, Vol. 12 (1975) at 471 and R. N. Schmidt, K. C. Park, and J. E. Janssen, xe2x80x9cHigh Temperature Solar Absorber Coatings,xe2x80x9d WPAFB Tech. Rep. No. ML-TDR-64-250, Part I (1963) and Part II (1964). Furthermore, the structure of the 5-layer and 7-layer coatings are significantly different than those used in previous studies. These coating designs have also been optimized to provide high surface emittance in the spectral range that matches the TPV cells (wavelength=1 to 3 microns) with low emittance at wavelength greater than 3 microns to improve efficiency, which is significantly different than the other coatings developed for solar applications with a high emissivity at wavelengths less than 1.0 microns.
This invention describes multilayer coatings for a TPV radiator that result in selective emissivity in the 1 to 3 micron range. Two three-layer embodiments are described. In the three-layer embodiments, a first oxide layer is placed on a radiator substrate. A second metal layer is placed over the first oxide layer. A third oxide layer is then placed over the second metal layer. In a particularly preferred embodiment, the radiator substrate is molybdenum, the first and third oxide layers are selected from the group consisting of Al2O3, TiO2, and HfO2 and are each approximately 10 to 2000 nm thick, and the second metal layer is molybdenum and is approximately 5 to 200 nm thick.
Two five-layer radiator coatings are disclosed. In the first embodiment, a first layer is placed on a radiator substrate, a second layer is placed over the first layer, a third layer is placed over the second layer, a fourth layer is placed over the third layer, and a fifth layer is placed over the fourth layer. In a particularly preferred embodiment, the radiator substrate is molybdenum. The first, third, and fifth layers are an oxide selected from the group consisting of Al2O3, TiO2, and HfO2 and are each approximately 10 to 2000 nm thick. The second and fourth layers are molybdenum and are each approximately 5 to 200 nm thick.
In a second five-layer coating embodiment, the first and fifth layers are a first oxide, which is an oxide selected from the group consisting of Al2O3, TiO2, and HfO2. Each layer is approximately 10 to 2000 nm thick. The second and fourth layers are a second oxide, which is an oxide selected from the group consisting of Al2O3, TiO2, and HfO2 and is different from the first oxide. Each of these layers is also approximately 10 to 2000 nm thick. The third layer is molybdenum and is approximately 5 to 200 nm thick.
Two seven-layer radiator coatings are disclosed. In the first embodiment, a first layer is placed on a radiator substrate, a second layer is placed over the first layer, a third layer is placed 5 over the second layer, a fourth layer is placed over the third layer, a fifth layer is placed over the fourth layer, a sixth layer is placed over the fifth layer, and a seventh layer is placed over the sixth layer. In a particularly preferred embodiment, the radiator substrate is molybdenum. The first, third, fifth, and seventh layers are an oxide selected from the group consisting of Al2O3, TiO2, and HfO2 and are each approximately 10 to 2000 nm thick. The second, fourth, and sixth layers are molybdenum and are each approximately 5 to 200 nm thick
In a second seven-layer coating embodiment, the first, third, fifth, and seventh layers are a first oxide, which is an oxide selected from the group consisting of Al2O3, TiO2, and HfO2. Each m layer is approximately 10 to 2000 nm thick. The second and sixth layers are a second oxide, which is an oxide selected from the group consisting of Al2O3, TiO2, and HfO2 and is different IFV from the first oxide. Each of these layers is also approximately 10 to 2000 nm thick. The fourth layer is molybdenum id is approximately 5 to 200 nm thick.
Accordingly, an object of this invention is to prepare TPV radiators that selectively emit photons in the range of 1 to 3 microns by coating the radiators with multiple layers of oxides and metals.